Annealing process for minimizing core loss in hot-rolled electric sheet segments



PROCESS FDR MINMIZING C RE Jakob BingeLBerlin, Germany No Drawing.Application January 11*, 1956 Sel'ialNm 558,604

' 1 "Claim. (Cl. 148-112) l 0... LOSS IN HOT-ROLLED ELECTRIC SHEET SEG-United States Patent C) ,gsilicon contents. 1.5 %-3.5'% Si).,.magneticand mecl 'apical -properties to be retained, which are notsimultanequslypresentin high silicon sheetsw Very: lowlosses areachieved with sheets, annealed in this way, and enhanced magnetization,Without the necessity of'starting with a. hard and brittlematerial withvreduced magnetization. The process does not demand the use of high gradelelectric furnace smelted' steel; sheets of cheap Siemens Martin steelare used. No use is made of grain orientation by cold rolling and. thuslow core losses and high magnctizationtinall directions of the sheet areachieved, for which reason the invention is particularly suitable forthe. treatment of the core material in generators and motors. As the newprocess does not depend on any metallurgical purification of the corematerial by quick flowing hydrogen withthe objectof decarburisationetc.,

it can be carried. out in: a static or creeping protective gas, that isin the simplest way under the purest conditions. a invention-isbased onnew discoveries in the, field of metal physics.

I Jynamo sheets with'silicon contentsabove 2% are at present stageof'technics annealed at about 800 C., for one thing to eliminate thedistortion hardenings arising during processing, for another to convertthe released carbon into graphite. The simplest method for this boxannealing isv in air. Box annealing at higher temperatures is deprecatedat the present stage of'technics, for at 1000 C. a deteriorationinthelossfigures and mechanical properties becomes noticeable, which isknown as over -ozoneattire...

With the present invention electric sheet segments, part c l y th se nto yn m e ec r machin can under 'certain conditions be annealed at muchhigher temperatures (1100'-'-1250 C.) with the object of reducing coreloss and enhancing magnetization.

It is, of course, known that transformer sheets are annealed at above1100 C. in a high vacuum, though improvements of quality are onlyachieved with a vacuum superior to 10- Torr. in consequence of a crystalpurification through pumping out the released and reaction gases fromthe sheets. This method of increasing the initial permeability isunsuitable for dynamo sheets, for in the vacuum during thehigh-temperature annealing a brittleness arises in the sheetsinsupportable for machines, which can not be removed, and again vacuumannealing is not a large-scale technical process.

In connection with the idea of the invention on the other hand it wasascertained on hot-rolled, normally annealed dynamo sheets with 2.4%silicon content after five hours re-annealing at 1200* C. in argon of 1atmosphere pressure that the hysteresis loss had sunk from 1.8

Patented June 9, 1959 .wJkg. (liflwattsikilogramme) to 1.2=w./kg. Thesiiniil= taneous metallographic test of the annealed material showed,besides a slight coarsening of the grain, the precipitation of. largeperlite islands, which in the micro- .structure of the original sheetcould not be discefnedeven in -fold' magnification.

The connection between the reduction of. hysteresis loss and theappearance of the perliteislands can only be interpreted as being theresult ofthe originally highly dispersed cementite having collectedduring high-temperature annealing and during the cooling stageprecipita-ting itself as coarse perlite. This. provides a new standpointinsofar as according to the foreign body theory agglomerations are lessharmful magnetically than highly dispersed distributions and so reducecoercive force and with it hysteresis loss. If following the abovementioned theory the coercive force H is represented in the form inwhich C is a constant and is the relation of the Blochwall""6 in theWeiss magnetic elementary area to the. size of, particle d, it will be.seen that with increasing coagulation of the foreign bodies the coerciveforce rapidly sinks.

In contrast to the usual view held by metallurgists, who regardelimination of carbon and other foreign. elements by chemical reactionsfrom the sheetas the sole prerequisite for reducing hysteresis loss,there is shown here the effect of a collective precipitation at thegrain bouna r g about. y dif us on du i g a eng hy an: neal, at 1200 C.

A h ame time it as Pos i le t pr v that, the silicon content withinnarrow limits plays a considerable pa t in prec p i n o ar as an nnealat he m temperature on low-silicon dynamo sheets produces a much smallerresult with regard to lowering hysteresis loss than .with high-siliconsheets. As it is knownthat l c n p m t s cry ta za on and. so acelerates the d splac men oi. th o eig e e ent .t th raln o m arics,theenhanced. nfl c of. h v s l con. on. collecti P c p i n n be. expla nd A fu n proc sse r a c w th. ing, emp r ur ann a ing. empera ure f. 2,5C. are rec mmen e for. lowilicon he s... for e y h ghli on:- shcets 1100C. so. on dyna o sheet with. .2%. silicon. c nent it Was. po sible toachieve a hysteresislossofOfZ w.-/ kg. atlZOOlC. after 1. hours...during. whi h t me monoc y tals of 1.0 di met r. ha fonnc lin h she As asult. of the anneali g. h mago t 'zafion l o increases at all fi'eldstrengths by 400 gausses.

The process can also be applied on segments which have been cut out fromunannealed panels, so long as the panels have been given an absolutelyeven surface by finishing.

As high-temperature annealing in a slowly flowing gas has the sameeffect as in a stagnant protective gas, it can be used as a technicalprocess, for the speed of flow can be reduced at Will so as tosubstitute a creeping gas flow for the stagnant gas.

In the light of the invention it is suggested that seg ments ofhot-rolled dynamo electric sheets should be annealed in slowly flowing,technically pure argon, hydrogen, or nitrogen at temperatures between1100 C. and 1250 C. for several hours.

The slow flow is a necessary condition, for otherwise undesirableforeign gases such as oxygen and water vapour would be drawn into thecontainer and oxidation ,high speed of flow between ferrosilicon alloysin chemically pure hydrogenywith 1100 C. and 1400" C. achieved highinitial permeabilities, which call for absolute purity of the material.It is known that decarburization in electric sheets at high temperaturescan only be achieved in quickly flowing hydrogen, for the reaction at ahigh temperature only becomes effective through constant disturbance ofthe equilibrium, whereas with slower flow the equilibrium is shifted farto the left and no decarburization takes place.

As a further result of the invention oxidation of the sheets over ashort period by oxygenous nitrogen can after cooling to 800 C. be ofparticular importance, in that by a chemical attack it again neutralizesthe brittleness of the sheets.

The practical operation of the process consists appropriately inarranging the sheet segments separated by Al O -powder, in severalstacks in an iron container welded or riveted hermetically; an inlet andan outlet pipe are welded to the container, through which the gas flows.The gases developed during heating up pass out through the outlet, whilethe inlet is closed. From 900 C. onward intermittent cleansing withargon is carried out for a short period, which can cease at 1200 C. whenthe container has been sealed. To counteract warping of the sheetsegments it is advisable to use weight plates suitably arranged in thestacks. A further artifice consists in applying a directed flow, drivingthe desired gas on to every point of the surface inside the sheet pile.for heating; it can, alternately with cooler hoods, be propped over thevarious containers on ceramic pillars, whereby a continuous annealingprogram can be maintained.

It is a matter of course that the annealing container can be usedrepeatedly when surrounded by a cheap protective gas and kept free fromscale.

There is today in electro-mechanical engineering a demand for dynamosheets with extremely low core loss. The direct way to achieve this isby re-annealing the stamped machine segments at 1100" to 1250 C.,' whichnot only eliminates cut-hardening but at the same time has an improvingelfect on the crystal structure of the iron. In contrast to transformercores, stamping reintroduces into the deeply incised machine segmentwith long teeth and wide guide holes, greater tensions, which ascut-hardening again enhances hysteresis loss. At the edges of the cutthe coercive force is greatly increased, on the other hand the meantooth induction is very high, so

Finally the hood furnace has proved useful that in segments with narrowteeth a rise in total core loss of the segment of 15%, in those with'widetceth of 10% is calculated. For this reason it has already beensuggested subjecting the stamped segments to a stress-relieving annealat 700 C.; this procedure is, however, not economical enough and forthat reason alone has never been introduced.

According to the invention, the total reduction in core loss is 45%. I iv This way of re-annealing stamped segments at very high temperaturecould only be proposed once it had been proved that the expansion due toheat is reversible and also that no appreciablechanges in length remainas a result of the precipitations caused and of the equalization oftension brought about. In practice accuracy of size of '-0.01 mm. intooth breadth and '-0.3 mm. in segment length is called for, for afterassembly a smooth slot surface must essentially be present to take theconductor.

With the aid of microscope micrometers with an accuracy to 1 micron, itwas established both on 200 mm. check gauges and on segments, thatannealing at 1200" C. causes a shrinkage of only 1.5-10- as long as thepanel has been normalized in the rolling mill at 800 C., and a shrinkageof 3 -10- without normalizing having been done. This proves thathigh-temperature annealing of segments does not influence the accuracyin size-of machine construction and the original dimensions are retainedin the slot within limits of '-0.01 mm.

I claim:

A heat treatment for stacks of hot-rolled silicon steel electric sheetsfor generators to produce segments with very low hysteresis losses andgood mechanical properties, which treatment comprises annealing readystamped segments with a silicon content of 1.5 %3.5% in an in dustrialprotective gas between 1100 and 1250" C. for several hours, and thencooling the stack to about 800 C., and then oxidizing the segments for ashort time in a stream of strongly oxidizing gas.

References Cited in the file of this patent UNITED STATES PATENTS 71,569,355 Cole Jan. 12, 1926 2,158,065 Cole et al May 16, 1939 2,209,684Crafts July 30, .1954 2,599,340

Littman et a1. -Q June 3, 1952 OTHER REFERENCES The Electric Journal,vol. 21, pages 55 to 61. Published in 1924 at Pittsburgh, Pa.

Stahl and Eisen: Vol. 48, pp. 484 and 485. Publ. in 1928. i y

Alloys of Iron and Silicon, 1st edition, pp. 355 .and 356. Edited byGreiner, Marsh and Stoughton. Published in 1933 by the McGraw-Hill BookCo., New'York,

Ferromagnetism, pp. 92 and 93. Edited by Bozorth. Published in 1951 byD. Van Nostrand Co.

